Search

The linear and non-linear dynamic response to an oscillatory shear flow of giant wormlike micelles consisting of Pb-Peo block copolymers is studied by means of Fourier transform rheology. Experiments are performed in the vicinity of the isotropic-nematic phase transition concentration, where the location of isotropic-nematic phase transition lines is determined independently. Strong shear-thinning behaviour is observed due to critical slowing down of orientational diffusion as a result of the vicinity of the isotropic- nematic spinodal. This severe shear-thinning behaviour is shown to result in gradient shear banding. Time-resolved Small angle neutron scattering experiments are used to obtain insight in the microscopic phenomena that underly the observed rheological response. An equation of motion for the order-parameter tensor and an expression of the stress tensor in terms of the order-parameter tensor are used to interpret the experimental data, both in the linear and non-linear regime. Scaling of the dynamic behaviour of the orientational order parameter and the stress is found when critical slowing down due to the vicinity of the isotropic-nematic spinodal is accounted for., Comment: Accepted by J. Phys.: Condens. Matter, CODEF II Special Issue. 20 pages, 9 figures

A systematic experimental study of dispersions of charged colloidal spheres is presented on the effect of steady shear flow on nucleation and crystal-growth rates. In addition, the non-equilibrium phase diagram as far as the melting line is concerned is measured. Shear flow is found to strongly affect induction times, crystal growth rates and the location of the melting line. The main findings are that (i) the crystal growth rate for a given concentration exhibits a maximum as a function of the shear rate, (ii) contrary to the monotonous increase of the growth rate with increasing concentration in the absence of flow, a maximum of the crystal growth rate as a function of concentration is observed for sheared systems, and (iii) the induction time for a given concentration exhibits a maximum as a function of the shear rate. These findings will be partly explained on a qualitative level., Comment: 17 pages, 10 figures, accepted in Langmuir

In the first part of this paper, an approximate theory is developed for the leading order concentration dependence of the sedimentation coefficient for rod-like colloids/polymers/macromolecules. To first order in volume fraction $\phi$ of rods, the sedimentation coefficient is written as $1+\alpha \phi$. For large aspect ratio L/D (L is the rod length, D it's thickness) $\alpha$ is found to very like $\propto (\frac{L}{D})^2/\log (\frac{L}{D})$. This theoretical prediction is compared to experimental results. In the second part, experiments on {\it fd}-virus are described, both in the isotropic and nematic phase. First order in concentration results for this very long and thin (semi-flexible) rod are in agreement with the above theoretical prediction. Sedimentation profiles for the nematic phase show two sedimentation fronts. This result indicates that the nematic phase becomes unstable with the respect to isotropic phase during sedimentation., Comment: Submitted to J. Chem. Phys. See related webpage http://www.elsie.brandeis.edu